1. Field of the Invention
The present invention relates to a support apparatus supporting a lens barrel (i.e., a lens barrel support), which has a socket (such as a tripod mount) produced by metal die casting, and which is revolvable about an optical axis with respect to the tripod mount.
2. Description of the Related Art
A relatively large lens barrel for, e.g., a super-telephoto lens often includes a fixed or detachable tripod mount or the like for securely holding an image taking apparatus to a tripod or a monopod. Also, the lens barrel is generally rotatable about an optical axis of a lens, i.e., revolvable, while the tripod mount is held stationary, to take an image in any of landscape- and portrait-oriented frames.
FIG. 5A illustrates a structure of a lens barrel provided with a general tripod mount produced by metal die casting. A lens barrel 1 is held rotatably with respect to a tripod mount 2 by fitting an annular fitting member 2a, which is a part of the tripod mount 2, into a groove 1a formed in a fixed barrel as a part of the lens barrel 1. As shown in FIG. 5B, after rotating the lens barrel 1 relative to the annular fitting member 2a fitted into the groove 1a of the lens barrel 1 to a predetermined position, a fastening screw 2b is tightened such that the annular fitting member 2a clamps the lens barrel 1 to fix the lens barrel 1 at the predetermined rotated position.
In order to allow the revolving operation, a slight play needs to be left at a portion where the lens barrel 1 and the tripod mount 2 are fitted to each other. Also, when the annular fitting member 2a is fitted and held in the groove 1a of the lens barrel 1, the position of the center of gravity of the lens barrel 1, including a camera (not shown) mounted thereto, in the direction of the optical axis sometimes differs from the position where the lens barrel 1 is supported to the tripod mount 2.
In such a case, if a force acting to tilt the lens barrel 1 is applied to a fitting portion of the tripod mount 2, contact pressure caused by end surfaces 2c of the annular fitting member 2a abutting against front and rear walls 1b of the groove 1a, which are located in planes perpendicular to the optical axis in the lens barrel 1, is increased in accordance with the principle of leverage, thus generating a large load of resistance to rotation.
To overcome the above-described problem, in Japanese Patent Laid-Open No. 10-082935, the front and rear end surfaces of the annular fitting member 2a are brought into point contact with the corresponding walls of the groove 1a of the lens barrel 1 to reduce the area of a frictional surface and to decrease the load of resistance to rotation.
FIG. 6A illustrates another example of a lens barrel provided with a tripod mount. A tripod mount 4 is fixed to a lens barrel 3 by using an annular fitting member 4a. A plurality of screwed shaft pins are attached to an outer peripheral surface of the lens barrel 3 at predetermined angles in the circumferential direction, and cylindrical retaining collars 5 are rotatably inserted over the screwed shaft pins. On the other hand, an inner peripheral groove 4b is formed inside the annular fitting member 4a, and guide grooves 4c are formed to extend perpendicularly to the inner peripheral groove 4b in continuation with it in the same number as the retaining collars 5 for guiding the retaining collars 5 to the inner peripheral groove 4b. 
The annular fitting member 4a is fitted to the lens barrel 3 by introducing the retaining collars 5, which are attached to the lens barrel 3, to the inner peripheral groove 4b through the guide grooves 4c, and by rotating the annular fitting member 4a to be engaged in the inner peripheral groove 4b. Thus, the lens barrel 3 is engaged with and supported to the tripod mount 3 in a state rotatable about the optical axis. By tightening a fastening screw 4d, the lens barrel 3 can be fixed to a desired rotated position.
Alternatively, instead of the guide grooves 4c, tooling holes can be bored in the lens barrel 3 from the outer peripheral side at positions coincident with the inner peripheral groove 4b in the direction of the optical axis. In this case, after fitting the annular fitting member 4a over the lens barrel 3, retaining collars 5 are inserted through the tooling holes and are assembled to the lens barrel 3 by using screwed shaft pins. The tooling holes are not exposed to the exterior in a finished state because they are concealed by another part after the assembling of the retaining collars 5.
The assembly method using the guide grooves 4c enables a user to detach the tripod mount 4 from the lens barrel 3, while the assembly method using the outer-peripheral tooling holes does not allow the user to detach the tripod mount 4 from the lens barrel 3.
The retaining collars 5 are selected to have a slight fitting play with respect to the width of the inner peripheral groove 4b of the tripod mount 4 so that the lens barrel 3 is smoothly rotatable without undergoing resistance. With the related art shown in FIGS. 6A and 6B, unlike the structure of FIGS. 5A and 5B in which the tripod mount 2 is fitted into the groove 1a of the lens barrel 1, a load imposed in the direction of the optical axis is generated as rotational friction. Accordingly, even when the position of the center of gravity of the lens barrel 3 in the direction of the optical axis fairly differs from the position where the lens barrel 3 is supported to the tripod mount 4, the operation of revolving the lens barrel 4 can be performed with a relatively small load.
The tripod mounts 2 and 4 need sufficient strength and are difficult to have a rotationally symmetrical shape. For that reason, the tripod mounts 2 and 4 are generally manufactured through the steps of producing a blank of each tripod mount by metal die casting, and forming the fitting portion (diameter), the engaging groove, etc. in match with a body of the lens barrel 1 or 3 by secondary working using a lathe.
In the case of the tripod mount 4 shown in FIGS. 6A and 6B, the inner peripheral groove 4b is formed by secondary working. FIG. 7 illustrates a finished shape of the annular fitting member 4a obtained by machining a blank of the tripod mount 4 shown in FIGS. 6A and 6B, which has been produced by metal die casting, to cut an unnecessary portion, indicated by dotted lines, of the annular fitting member 4a with a lathe, thus forming the inner peripheral groove 4b by the secondary working.
In general metal die casting, however, when a die casting material is poured into a die, a slight amount of air is entrained with the die casting material in some cases. Accordingly, porosities (cavities) are often generated in a central region of a thick portion where the applied casting pressure tends to be insufficient. This increases a possibility that porosities appear on the cut surface when a cutting amount by the secondary working is large as in the case of machining the inner peripheral groove 4b shown in FIG. 7.
If porosities are exposed to the surface of the inner peripheral groove 4b, i.e., its sliding surface in contact with the retaining collars 5, as in the example shown in FIGS. 6A and 6B, a feeling of the revolving operation deteriorates upon the retaining collars 5 sliding against the porosities when the les barrel 3 is revolved.
Another problem is that, because the annular fitting member 4a has insufficient strength and is apt to brittle at locations where the porosities are generated, the sliding surface is apt to crumble with repeated sliding movements, thus generating wear debris.
A metal die casting method for making porosities hard to generate inside a die-cast product (i.e., a die cast) is also known as, for example, a vacuum die casting method of pouring a die casting material into a die after the interior of the die has been evacuated to a vacuum state, or a PF (Pore Free) method of filling the interior of the die with active oxygen. However, those methods have the problems of needing large man-hours and increasing the cost.